1. Field of the Invention
This invention relates to an apparatus and method for punching holes in thin sheet material and, more particularly, to a die used with a gang-punch apparatus for punching holes in a desired hole pattern in a greensheet used in the manufacture of electronic components.
2. Description of Related Art
In the manufacture of multilayer ceramic (MLC) substrates for integrated circuit semiconductor package structures, a plurality of green ceramic sheets (commonly termed greensheets) are formed by doctor blading a slurry containing a resin binder, a particulate ceramic material, solvents and a plasticizer, drying the doctor bladed sheet and cutting it into appropriate size sheets. Through-holes, commonly termed vias, are then punched in the greensheets to facilitate forming electrical interconnections through the sheet. Electrically conductive paste is deposited in the holes and in appropriate patterns on the surface of each of the greensheets used to make the MLC substrate, the sheets stacked and laminated, and the assembly subsequently fired at a sintering temperature to form the MLC component.
The formation of the vias in the ceramic material is accomplished in its green state. Prior to being fired, the greensheets are easily deformed and the precision with which the via holes are formed is greatly dependent upon the use of an apparatus which minimizes distortion or disruption to the structural integrity of the greensheet other than forming the via hole. With the size of electrical components constantly being reduced through technical improvement, the positioning of the vias in ceramic greensheets is becoming ever more restrictive. Further, with the increase in the quantity of the electronic circuits that may be positioned and packaged on a small area of the ceramic substrate, a requirement for a larger number of via holes in the greensheet is evident.
Large numbers of holes must be punched into a greensheet with a high degree of precision in order to properly position the holes. At the same time, this punching operation must occur very rapidly in order to produce the large numbers of greensheets that are necessary in the mass production of electronic components.
One method for punching greensheets is using a gang-punching apparatus. A gang-punch is a punch/die set where the punches for a particular ceramic greensheet layer are mounted in a fixed plate. The gang-punch is used to punch the entire greensheet (layer) with a single stroke. A gang-punch has very high throughput and is cost effective for very high volume products. Different gang-punches however are needed to punch each different greensheet layer.
A number of attempts have been made to provide a universal gang-punch apparatus which will economically and efficiently provide the means to make via openings in any pattern on a greensheet. One type apparatus utilizes an interposer mask which contains hole openings where holes are desired to be punched. In these apparatus, a plurality of punch elements arranged in a grid is positioned over the greensheet which is covered by the interposer mask. The die has openings corresponding to all the punch elements. When the punch elements contact the interposer mask as the punch heads are moved downwardly, the holes will be punched where the openings occur since the punch elements would pass through the openings in the interposer mask and through the ceramic greensheet. In other areas covered by the interposer mask, i.e., where holes are not desired, the interposer mask will cause the punch element to be retracted into the punch head. A progressive gang-punch utilizes steel balls on the surface of the punch head at locations of punches to be punched which when a roller traverse the top of the punch head only vias are punched where steel balls are located.
Basically, the gang-punch apparatus uses punch pins mounted in the gang-punch head and the punch head apparatus is placed over a greensheet which is disposed on the surface of a die. The die has openings corresponding to all the punches in the punch apparatus and when the punch apparatus is actuated the punch pins are forced downward and outward from the punch apparatus through the greensheet and through the corresponding openings in the die where the selective holes are desired to be punched. For vias not to be punched, those punch pins in the gang-punch head are not actuated and the desired pattern is obtained. The die necessarily contains corresponding openings for all the punch pins in the gang-punch head so that the desired vias may be formed in any greensheet. A preferred gang-punch apparatus is shown in U.S. Pat. No. 6,223,636, which patent is hereby incorporated by reference.
In operation, punching of the greensheet vias produce a round piece of greensheet material corresponding to the punch size and thickness of the greensheet and this punched material, typically called a slug, is forced downward through the die openings and is removed from the die. Unfortunately however, the slugs may stick and not fall through the die openings and over time may cause a blockage in the die. Continued punching forces the slugs to compact in the die until the punch in the packed hole breaks. The punch head must then be removed and the broken punch replaced causing costly tool downtime.
A gang-punch is also a high-speed punch apparatus capable of punching an entire greensheet every few seconds regardless of the number of vias. Due to the high usage of the gang-punch apparatus, which may be used to punch as many as 10,000 greensheets per day, the die wears rapidly and the die holes wear past there process specification and can no longer be used. One method to increase die life is to apply coatings to the die but this solution has not been successful since the coating distorts the die. It was also found that the coatings wore rapidly around the cutting edge of the die and eventually wore past their process limits and/or cracked and flaked off.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a die for use with a gang-punch apparatus wherein the slugs formed during the punching operation easily fall through the die and are removed from the die.
It is another object of the present invention to provide a method of punching selective hole patterns in thin sheet materials such as a greensheet using a punch apparatus, in particular, a gang-punch apparatus, wherein slugs formed during the punching operation easily fall through the die and are removed from the die.
Another object of the present invention is to provide a die which has a long operating life and is inexpensive to manufacture with close tolerances of about xc2x10.0001 inch in hole diameter and hole location is easy to replace quickly and has a top die sheet which is disposable.
Another object of the present invention is to provide a method of forming vias in greensheets using the improved die of the invention.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, a die which is used with a punch apparatus, particularly a gang-punch apparatus, comprising:
a die support frame having a peripheral edge containing one or more vacuum manifolds with the center of the frame being open;
a groove plate positioned in the die support frame opening and having a plurality of longitudinal channels formed by a plurality of vertical longitudinal supports extending from one side of the groove plate to the other side of the groove plate, the groove plate being substantially the same height as the die support frame;
a window plate disposed on the groove plate and the die support frame comprising a plurality of vertical through-holes, each through hole corresponding to a punch of a punch apparatus and a plurality of the holes having an oval shape with a long axis and a short axis with the long axis being preferably transverse to the longitudinal supports and a number of through openings at the periphery of the plate communicating with the vacuum manifold of the die support frame; and
a die plate disposed on the window plate and having vertical through-holes corresponding to punches of a gang-punch apparatus and of the through-holes of the window plate.
In another aspect of the invention the die has a plurality of registration through-holes in each of the plates forming the die to align the die apparatus.
In a further aspect of the invention the window plate has a central area with cross-supports forming a grid with open areas, with the remainder of the window plate outside the central area having the vertical through-holes as discussed above.
In a preferred die apparatus of the invention the die plate is formed from at least two sheets. One sheet is an upper die plate having through-holes corresponding to punch locations in the gang-punch apparatus and of the window plate and which die plate is formed from a wear-resistant material such as molybdenum. The upper die plate is preferably made using photolithographic technologies and is disposable. The other lower sheet is a sub-die plate having corresponding through-holes which are preferably slightly larger than the through-holes in the die plate and openings in the periphery thereof communicating with the vacuum manifold of the die support frame. The sub-die plate is preferably formed from a material such as stainless steel.
In another preferred die of the invention the sub-die plate has a cut-out indent (center-bore) adjacent the registration holes for positioning of the upper die plate on the sub-die plate.
In another aspect of the invention a method is provided for gang punching selective hole patterns in thin sheet materials such as greensheets comprising the steps of:
providing a gang-punch apparatus whereby when the gang-punch apparatus is actuated a number of punches are forced outward and downward from the apparatus;
supplying a die as described above;
supplying a greensheet disposed on top of the die; and
actuating the gang-punch apparatus to force punch pins from the gang-punch apparatus through the greensheet into the die forming openings in the greensheet and forming slugs or punched greensheet material which slugs fall through the die and are removed from the die channels.